Ultrafast Vibrational Sum Frequency Dynamics of SCN^- and the Impact of Interfacial Hydrogen-Bonding Networks on the Vibrational Density of States

The vibrational density of states (VDOS) provides insight about energy dissipation and the intermolecular network of the medium. The quantification of the VDOS at interfaces mostly relies on Molecular Dynamics (MD) simulations as experimental analysis of the VDOS at an interface is still very challenging. Here, we use the vibrational lifetime of SCN^-to probe the VDOS of the O-D stretching region for two model interfaces: D₂O/Al₂O₃(0001) and D₂O/Al₂O₃(110). The Al₂O₃(0001) surface is atomically smooth with hydroxyls on the surface pointing in-plane and out-of-plane, while the Al₂O₃(110) surface is rougher with a greater distribution of surface hydroxyl types and orientatons. MD simulations reveal distinctions in the VDOS associated with the O-D stretching between these two interfaces and their hydrogen-bonding networks. Steady-state vibrational sum frequency generation (vSFG) spectroscopy further confirms that the O-D stretching exhibits greater intensity in the SCN^- stretching region of D₂O/Al₂O₃(0001) compared to D₂O/Al₂O₃(110). IR pump/vibrational SFG probe experiments reveal that SCN^- has a shorter vibrational lifetime at the D₂O/Al₂O₃(0001) interface than at the D₂O/Al₂O₃(110) interface. This indicates a greater coupling between the CN stretching of SCN^-and the O-D stretching at D₂O/Al₂O₃(0001), likely due to a higher VDOS of O-D stretching at D₂O/Al₂O₃(0001) interface than the D₂O/Al₂O₃(110), accelerating the SCN^- vibrational relaxation process. This study highlights the effectiveness of SCN^- as a probe molecule in exploring the VDOS differences for the O-D stretching and hydrogen-bonding environment at chemically and physically distinct aqueous/sapphire interfaces.